Network systems enable communication of messages among computer systems. For example, network systems enable communication of files over the Internet. Increases in computer and Internet usage have resulted in an increased number of files exchanged, causing network resources to become increasingly taxed and difficult to operate and maintain. To complicate matters, attachments may be included with files being exchanged over a network, leading to the dedication of additional network resources to the communication and storage of particular files. In fact, a popular file (e.g., electronic mail message) and its attachment may be sent numerous times from a single source or from subsequent recipients of the file and its attachment.